In the state of the art, functional coatings on materials are achieved by industrial polymerization processes that consist of multiple processing steps of the material to be coated, including one step of deposition by spreading or spray-coating of precursors containing polymerization initiators and one step of (radical or ionic polymerization) polymerization, that is obtained through thermal or ultraviolet (UV) energy or by electron beam (EB) radiation.
In any case chemical initiators are used, that facilitate polymerization and add stability to the deposit. The use of polymerization initiators is required, for example, for radical polymerization using UV systems and for ionic polymerization using either UV or EB systems. The amount of photoinitiator in the formulation, as a function of the system to be selected, ranges from 0.5 to 15%. Besides being very expensive, photoinitiators are toxic for humans, whereby systems that can polymerize without their assistance are highly desired.
Processes of this type are disclosed, for instance in U.S. Pat. No. 6,268,403 and EP412430.
Furthermore, in case of a radical polymerization process, (thermal, UV or EB) polymerization must occur without oxygen. Oxygen is a strong inhibitor of this type of polymerization, and currently used methods to prevent such contamination include inert atmosphere curing (under nitrogen atmosphere), the use of particular photoinitiators, the use of increased intensity of UV radiation, the use of oxygen scavengers, the use of waxes and shielding films. As far as UV polymerization is concerned, the addition of waxes or other airtight compounds are known to reduce the inhibitory effect of oxygen. In order to improve surface curing in high rate processes, the concept of a barrier against oxygen has been implemented resulting in a technology consisting of applying a gelatin layer containing a high-concentration of initiator on the formulation to be polymerized. Besides preventing diffusion of oxygen from the surrounding environment, this surface layer can provide an additional amount of initiator during UV irradiation. Moreover, since gelatin is not polymerizable under UV irradiation, the layer may be easily removed by water rinsing after curing. The results obtained using wax-based barriers are similar to those obtained in a controlled inert atmosphere of inert gas (e.g. nitrogen).
Alternative methods for surface modification of materials under testing use vacuum or atmospheric-pressure plasmas. Particularly, plasmas can be used to obtain coatings of various thicknesses (of the order of one micron or less) with other functional properties. The material treatment process consists in mixing the gas- or vapor-phase precursor in a gas, typically a noble or inert gas, creating a plasma from such mixture for fragmentation and dissociation of the molecules of the precursors to obtain chemical reactive groups to be deposited on the substrate.
The deposition process is carried out in most cases through a radical polymerization step that requires an oxygen-free environment.
Vacuum processes have the advantage that they are carried out in a controlled-pressure environment and with inert gases with very low oxygen contamination, but use expensive vacuum equipment and chambers in which these treatments may be performed in a roll-to-roll configuration.
At atmospheric pressure the oxygen contamination problem may be obviated by placing the plasma source in an appropriate chamber simulating a closed and controlled environment, in which through-apertures allow continuous material treatment in an oxygen-free environment. Atmospheric plasma is necessarily produced in a mixture of inert or noble gas, typically nitrogen, which acts as a carrier gas of the precursor to be polymerized.
WO 02/28548 discloses a method of depositing functional coatings by combining an atmospheric-pressure discharge plasma and an atomized precursor. This will allow deposition of a series of coatings, that preserve most of the functionality of the monomer being used.
WO 2003/085693 discloses an atmospheric pressure plasma process, in which a process gas and an atomized precursor are introduced into the plasma region between the electrodes.
U.S. Pat. No. 8,178,168 discloses a method for depositing polymeric coatings, wherein a mixture containing a radically polymerizable monomer and a radical initiator undergo an atmospheric pressure plasma treatment and the resulting polymeric coating is deposited on a substrate. The plasma is preferably generated by inert gases (argon, helium) or mixtures thereof with other gases (air, nitrogen, oxygen, ammonia, water vapor). The substrate may be previously activated by means of another atmospheric pressure plasma treatment.
The possibility of depositing functional coatings on various substrates through multi-step processes (low-pressure plasma pre-activation—monomer impregnation—low-pressure plasma treatment) is well documented in literature. See for instance the following scientific publications: M. J. Tszfack et al, Surface & Coating Technology 200 (2006) 3503-3510. All plasma treatments are conducted in closed low-pressure chambers, using inert gases, and hence under very low oxygen-contamination conditions.
Similar procedures are also carried out using atmospheric pressure plasma treatments (see for instance C. Chaiwong et al., Surface & Coating Technology 204 (2010) 2991-2995). Nevertheless, also in this case the gas that is used to produce the plasma shall be an inert or noble gas, to avoid the presence of oxygen, which would inhibit the radical polymerization process.
“Stability Study of Polyacrylic Acid Films Plasma Polymerized on Polypropylene Substrate at Medium Pressure” vol. 257 No. 2, 1 Nov. 2010 pages 372-380 Applied Surface Science Elsevier Amsterdam NL, describes a process for coating a product for biomedical use by polymerization of acrylic acid with inert gas at a pressure close to atmospheric pressure.
On the other hand, WO 2003/089479 discloses a method for coating a substrate by deposition of a monomer composition containing a mixture of ionically and/or radically polymerizable monomers and successive plasma treatment.
Nevertheless, this process requires the presence of a radical photocatalyst, if polymerization occurs by a radical mechanism and a ionic catalyst if polymerization occurs by a ionic mechanism, and such components have already been mentioned to contaminate the coating and the final coated material. Furthermore, also in this process plasma treatment is conducted under vacuum or with inert gases.
U.S. Pat. No. 5,580,606 discloses a process for coating a substrate made of a plastic material, comprising the following steps:                a) deposition of a polymerizable composition on said plastic material, the composition containing a silane with methacryloxy or vinyl functional groups, and a polyfunctional epoxy compound as well as a curing agent, with the use of photocatalysts;        b) vacuum plasma treatment (0.1-0.14 mbar).        
Therefore, the need exists for a substantially photocatalyst-free process for coating a substrate with a polymeric film.
Also, the need exists for a process for coating a substrate with a polymeric film that is substantially simple, easily scalable and suitable for application in continuous industrial coating processes.